An electric double layer capacitor for repeated use by charging is known as a capacitor for storing electric charge in an adsorbed layer of ions formed in pores of a porous carbon electrode such as activated carbon, i.e., an electric double layer. With longer life and higher output, the electric double layer capacitor has been widely used as a power source for computer memory backup, particularly as a power storage system in railroad vehicles and an auxiliary power in hybrid vehicles.
Nowadays, to improve the energy density of an electric double layer capacitor, hybrid capacitors, which use not only an activated carbon electrode but also an active material of a secondary cell as an electrode material, are being developed. Illustrative example thereof includes a lithium ion capacitor. The lithium ion capacitor includes an activated carbon on a positive electrode, a carbon material for a negative electrode of a lithium ion cell on a negative electrode, and an organic electrolyte for a lithium ion cell as an electrolyte.
As shown in FIG. 24, an electric double layer capacitor is charged by connecting a power source between activated carbon electrodes of a positive electrode and a negative electrode immersed with an electrolyte to apply voltage. During charging, electrolyte ions adsorb on a surface of each electrode. Specifically, negative ions (−) in the electrolyte are attracted to positive holes (h+) of the positive electrode, and positive ions (+) in the electrolyte are attracted to electrons (e−) of the negative electrode. Both combinations of a positive hole (h+) and a negative ion (−), and an electron (e−) and a positive ion (+) are oriented at an extremely small distance of approximately few Å to form an electric double layer. This state is kept, even with the power source being disconnected, and the storage state is maintained without using any chemical reaction. During discharging, positive ions and negative ions which have been adsorbed to respective electrode desorb therefrom. Specifically, return of the electrons (e−) to the positive electrode leads to fewer positive holes (h+) and then diffusion of positive ions and negative ions in the electrolyte again. Consequently, since both charging and discharging processes cause no change on a capacitor material, heat generation or degradation by chemical reaction is not found to ensure longer life of the capacitor.
An electric double layer capacitor is characterized by technological advantages over a secondary cell: (1) capability of charging and discharging at a high rate, (2) high reversibility of charging and discharging cycles, (3) longer cycle life, and (4) environmentally friendly property due to no use of heavy metal in an electrode or an electrolyte. These characteristics are associated with no use of heavy metal in an electric double layer capacitor, operation by ion's physical adsorption and desorption, and no electron transfer reaction of chemical species.
Since the energy (E) stored in an electric double layer capacitor increases in proportion to the product of the square of the charging voltage (V) and the electric double layer capacitance (C) (E=CV2/2), energy density is effectively improved by improvement in capacitance and charging voltage. The charging voltage of the electric double layer capacitor is normally suppressed to about 2.5V. This is primarily because charging with a voltage of 3V or more starts electrolysis between electrodes and an electrolyte to reduce the capacitance and degrades the electric double layer capacitor.
A practical activated carbon for use in an electrode of an electric double layer capacitor is currently produced, as shown in FIG. 25, by adding an appropriate amount of a conductive auxiliary agent such as carbon black to activated carbon particles with a diameter of 1 to 10 μm and molding the same into a sheet shape using a fibrillated binder such as polytetrafluoroethylene. It can be thought that a decline in capacitance of the electric double layer capacitor when charged with a voltage of 3V or more is caused not only by an activated carbon or an electrolyte, but also by a binder comprising an activated carbon for use in an electrode or a conductive auxiliary agent.
To achieve a higher capacitance of an electric double layer capacitor, which, however, doesn't solve the above capacitance reduction, an activated carbon for use in an electrode containing no binder or conductive material, or a seamless activated carbon for use in an electrode in which no contact interface between activated carbon particles is found has been proposed (e.g., see Non-patent document 1). Non-patent document 1 discloses a method for directly preparing an activated carbon for use in an electrode without using a binder by using a characteristic of a sol-gel process excellent in moldability. The capacitance of an electrode composed of an activated carbon prepared by the above method (binder-free electrode) is higher than that of an activated carbon prepared by using a binder. It was also confirmed that the thicker an activated carbon for use in an electrode is, the more remarkable difference in capacitance becomes.
A carbon material with a specific surface area of about 1000 m2/g having a microporous structure prepared by firing a polyacrylonitrile polymer (PAN) porous body is disclosed (see e.g. Non-patent document 2) for another seamless activated carbon for use in an electrode. An activated carbon for use in an electrode shown in Non-patent document 2 is produced by dissolving PAN in a mixed solvent of dimethyl sulfoxide and water by heating and agitating the same, heating a cooled molded object in the air at 230° C. for 1 hour, and further heating the same in carbon dioxide/argon atmosphere at 900° C. for 2 hours.
Further, another method for producing an activated carbon for use in an electrode is to mold a tablet-shaped carbon material without using a binder (see e.g. Patent document 1). Specifically, Patent document 1, a tablet-shaped carbon material is produced by reacting a phenolic compound and an aldehyde compound in a disk-shaped vessel in the presence of water and a catalyst to obtain a tablet-shaped wet gel, substituting water in the wet gel with a hydrophilic organic solvent and freezing dry the wet gel to obtain a tablet-shaped dried gel, and firing the tablet-shaped dried gel in inert atmosphere. A tablet-shaped carbon material molded without using a binder by the method includes a microstructure such as a micropore with a diameter of under 2 nm and a mesopore (a pore with a diameter of 2 to 50 nm).
Also, a block of an activated carbonized resin porous body substantively having a continuous pore structure which includes open pores is disclosed (see e.g. Patent document 2). Herein, an activated carbon block obtained by carbonizing and activating a phenolic resin molded object is disclosed as a preferable example.